The present invention relates to an improvement in automatic choking of a variable venturi carburetor in which the effective cross-sectional area of a venturi portion is varied in correspondence to the quantity of air sucked in.
In a conventional variable venturi carburetor such as a so-called SU carburetor, a variable venturi portion is provided upstream of a throttle valve, and the venturi is driven by a vacuum-responsive device so that its degree of opening is controlled by a feedback system, so as to maintain the vacuum value between the variable venturi and the throttle valve substantially constant. With the increase in the venturi opening, therefore, the effective area of the fuel nozzle which supplies fuel to the venturi portion should also be increased, so as to control the fuel flow rate to be proportional to the air flow rate and to control the air/fuel ratio to be always constant. This type of carburetor has good stability performance, and particulates and atomizes the fuel well. However, because its air/fuel ratio is inherently constant, a special mechanism must be provided for supplying air/fuel ratio of higher fuel concentration, on occasions when such is required, such as starting of the engine, warming up of the engine, and acceleration of the engine before the end of warming up of the engine.
It has been formerly proposed to move the position of the jet orifice which is part of the metering jet portion of the fuel supply system, during starting and warming up, so as to enlarge the aperture between this jet orifice and a metering needle inserted thereinto and controlled by the variable venturi, so as to enrich the mixture at these times. Simultaneously the opening of the throttle valve is performed by a cam mechanism, so as to ensure the necessary amount of choking mixture. However, such a device has many problems, such as prevention of oil leakage from the jet moving device, precision positioning of the jet, providing operating power for the cam and the jet mechanism, and so forth; and it is deficient, in that the precision of control of the air/fuel ratio and air flow is insufficeint.
Further, there has also been proposed a device wherein a choke valve air passage is set up in parallel to the venturi portion, the effective cross-sectional area of this choke valve air passage being reduced during starting and warming up of the engine, so as to raise the vacuum value downstream of the variable venturi to a higher value than normal at these times. Thus the amount of fuel sucked in from the fuel nozzle is increased, so as to give the required starting and warming up mixture. This system however does not attack the problem of need for richer mixture during acceleration before the engine is fully warmed up (nor does the first-explained system), and such a type of variable venturi carburetor comes to have as many as three choking mechanisms, which is undue complication.
It has also been proposed to provide a choke valve mechanism as in conventional carburetors, which raises the negative pressure on the main nozzle above the level used in normal operation, whereby the fuel supply is increased. Because automatic manipulation of the amount of opening of such a choke valve is required in correspondence to atmospheric and engine temperature, extra devices such as a bimetallic element, a heater, an ignition diaphragm, and a dynamic pressure system for sucked air, etc., must be provided, which again means that the final carburetor is costly and bulky, and also becomes hard to adjust correctly and to service. Again, this solution does not attach the need for richening of mixture during acceleration before full warming up of the engine.